modules: lvgl: Fix stdlib usage

LVGL already provides an implementation wrapping a C standard library into
its own lv_* functions. Add them to the CMakeLists and set the appropriate
defines for them.

Signed-off-by: Fabian Blatz <fabianblatz@gmail.com>
(cherry picked from commit 076994f49225779ff8ccdc9d3be13663d4096405)
2 files changed